Nickel alloys containing significant quantities of chromium and molybdenum have been used by the chemical process and allied industries for over eighty years. Not only can they withstand a wide range of chemical solutions, they also resist chloride-induced pitting, crevice corrosion, and stress corrosion cracking (insidious and unpredictable forms of attack, to which the stainless steels are prone).
The first nickel-chromium-molybdenum (Ni—Cr—Mo) alloys were discovered by Franks (U.S. Pat. No. 1,836,317) in the early 1930's. His alloys, which contained some iron, tungsten, and impurities such as carbon and silicon, were found to resist a wide range of corrosive chemicals. We now know that this is because molybdenum greatly enhances the resistance of nickel under active corrosion conditions (for example, in pure hydrochloric acid), while chromium helps establish protective, passive films under oxidizing conditions. The first commercial material (HASTELLOY C alloy, containing about 16 wt. % Cr and 16 wt. % Mo) was initially used in the cast (plus annealed) condition; annealed wrought products followed in the 1940's.
By the mid-1960's, melting and wrought processing technologies had improved to the point where wrought products with low carbon and low silicon contents were possible. These partially solved the problem of supersaturation of the alloys with silicon and carbon, and the resulting strong driving force for nucleation and growth of grain boundary carbides and/or intermetallics (i.e. sensitization) during welding, followed by preferential attack of the grain boundaries in certain environments. The first commercial material for which there were significantly reduced welding concerns was HASTELLOY C-276 alloy (again with about 16 wt. % Cr and 16 wt. % Mo), covered by U.S. Pat. No. 3,203,792 (Scheil).
To reduce the tendency for grain boundary precipitation of carbides and/or intermetallics still further, HASTELLOY C-4 alloy (U.S. Pat. No. 4,080,201, Hodge et al.) was introduced in the late 1970's. Unlike C and C-276 alloys, both of which had deliberate, substantial iron (Fe) and tungsten (W) contents, C-4 alloy was essentially a very stable (16 wt. % Cr/16 wt. % Mo) Ni—Cr—Mo ternary system, with some minor additions (notably aluminum and manganese) for control of sulfur and oxygen during melting, and a small titanium addition to tie up any carbon or nitrogen in the form of primary (intragranular) MC, MN, or M(C,N) precipitates.
By the early 1980's, it became evident that many applications of C-276 alloy (notably linings of flue gas desulfurization systems in fossil fuel power plants) involve corrosive solutions of an oxidizing nature, and that a wrought, Ni—Cr—Mo alloy with a higher chromium content might be advantageous. Thus, HASTELLOY C-22 alloy (U.S. Pat. No. 4,533,414, Asphahani), containing about 22 wt. % Cr and 13 wt. % Mo (plus 3 wt. % W) was introduced.
This was followed in the late 1980's and 1990's by other high-chromium, Ni—Cr—Mo materials, notably Alloy 59 (U.S. Pat. No. 4,906,437, Heubner et al.), INCONEL 686 alloy (U.S. Pat. No. 5,019,184, Crum et al.), and HASTELLOY C-2000 alloy (U.S. Pat. No. 6,280,540, Crook). Both Alloy 59 and C-2000 alloy contain 23 wt. % Cr and 16 wt. % Mo (but no tungsten); C-2000 alloy differs from other Ni—Cr—Mo alloys in that it has a small copper addition.
The design philosophy behind the Ni—Cr—Mo system has been to strike a balance between maximizing the contents of beneficial elements (in particular chromium and molybdenum), while maintaining a single, face-centered cubic atomic structure (gamma phase), which has been thought to be optimum for corrosion performance. In other words, designers of the Ni—Cr—Mo alloys have been mindful of the solubility limits of possible beneficial elements and have tried to stay close to these limits. To enable contents just slightly above the solubility limits, advantage has been taken of the fact that these alloys are generally solution annealed and rapidly quenched, prior to use. The logic has been that any second phases (that might occur during solidification and/or wrought processing) will be dissolved in the gamma solid solution during annealing, and that the resultant single atomic structure will be frozen in place by the rapid quenching. Indeed, U.S. Pat. No. 5,019,184 (for INCONEL 686 alloy) goes so far as to describe a double homogenization treatment during wrought processing, to ensure a single (gamma) phase structure after annealing and quenching.
The problem with this approach is that any subsequent thermal cycles, such as those experienced during welding, can cause second phase precipitation in grain boundaries (i.e. sensitization). The driving force for this sensitization is proportional to the amount of over-alloying, or super-saturation.
Pertinent to the present invention is work published in 1984 by M. Raghavan et al (Metallurgical Transactions, Volume 15A [1984], pages 783-792). In this work, several nickel-based alloys of widely varying chromium and molybdenum contents were made in the form of cast buttons (i.e. not subjected to wrought processing), for study of the phases possible under equilibrium conditions, at different temperatures in this system, one being a pure 60 wt. % Ni-20 wt. % Cr-20 wt. % Mo alloy.
Also pertinent to the present invention is European Patent EP 0991788 (Heubner and Köhler), which describes a nitrogen-bearing, nickel-chromium-molybdenum alloy, in which the chromium ranges from 20.0 to 23.0 wt. %, and the molybdenum ranges from 18.5 to 21.0 wt. %. The nitrogen content of the alloys claimed in EP 0991788 is 0.05 to 0.15 wt. %. The characteristics of a commercial material conforming to the claims of EP 0991788 were described in a 2013 paper (published in the proceedings of CORROSION 2013, NACE International, Paper 2325). Interestingly, the annealed microstructure of this material was typical of a single phase Ni—Cr—Mo alloy.